Recently, research has been conducted on pico cells for distributing the load of a macro cell in the 3GPP. The heterogeneous network environment where macro and pico cells coexist has come into the spotlight because it offers the possibility of improving system performance in comparison with the legacy macro cell environment.
The 3GPP has proposed an enhanced Inter-Cell Interference Coordination (eICIC) (or time-domain ICIC) technique for balancing the load between the macro and pico cells more effectively.
Since it is typical that a pico cell with low transmission power has a short antenna in comparison with the macro cell, it is difficult to balance the load between the macro and pico cells efficiently, especially when using an eNB-UE association rule (rule for the eNB with the highest signal strength to serve the UE).
That is, if the UEs select the cell with the highest Reference Signal Received Power (RSRP) as their serving cell, a certain UE may connect to the macro cell even when a pico cell is the best cell. Such a UE is likely to cause interference within the pico cell and thus degrade total network throughput. Also, if the number of UEs connected to the macro cell is much smaller than the number of UEs connected to the pico cell, this is very inefficient in view of resource utilization.
In order to solve the above problems, the eICIC uses a Cell Range Expansion (CRE) technique for providing the UE with criteria for handover between macro and pico cells. That is, the eICIC is designed such that the UE selects the pico cell from which the signal strength is greater than a value acquired by reflecting a CRE bias (unit: dB) to the signal strength from the macro cell as the serving cell of the UE in order to improve the load balancing effect.
However, the UEs that have connected to the pico cell due to the CRE bias may suffer interference from the macro cell because the RSRP from the macro cell is still stronger than that of the pico cell.